2. Florida state Senate President Mike Haridopolos: Haridopolos looked like a promising candidate to take on Sen. Bill Nelson, D-Fla., when he announced earlier this year -- and he started with a bang, raising a whopping $2.6 million in the first quarter of his bid, more than any other Senate challenger that quarter. But that turned out to be the high point of his campaign.
Over the next few months, the campaign had a string of gaffes and bad headlines, with a controversy over payment for a book he wrote, a campaign leadership shake-up, an ethics issue in the Legislature, and an inability to articulate his position on Rep. Paul Ryan's, R-Wis., plan. Haridopolos ended his Senate bid in July
1. Sen. Ben Nelson, D-Neb.: In the last week of 2011, Nelson announced he wouldn't run for reelection. In a year when the Democrats encouraged senators up for reelection to make early decisions on retirements, he instead waited until after Democratic groups poured over $1 million into running ads on his behalf to announce his decision. Even if Nelson had run, it would have been a tough seat to hold - but with him out of the running, it's very likely the seat will flip to the GOP in 2012.